Investments (Distribution Of Securities By Percentage Of Fair Value Below Book Value By More Than 5%) (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012 Security
>5% to 15% [Member]
Schedule of Investments [Line Items]
Number of Securities	31
Book Value	$ 309
Fair Value	278
Percentage Book Value Exceeded Market Value, Minimum	5.00%
Percentage Book Value Exceeded Market Value, Maximum	15.00%
>15% to 25% [Member]
Schedule of Investments [Line Items]
Number of Securities	18
Book Value	682
Fair Value	549
Percentage Book Value Exceeded Market Value, Minimum	15.00%
Percentage Book Value Exceeded Market Value, Maximum	25.00%
>25% to 50% [Member]
Schedule of Investments [Line Items]
Number of Securities	13
Book Value	69
Fair Value	45
Percentage Book Value Exceeded Market Value, Minimum	25.00%
Percentage Book Value Exceeded Market Value, Maximum	50.00%
> 50% [Member]
Schedule of Investments [Line Items]
Number of Securities	27
Book Value	87
Fair Value	18
Percentage Book Value Exceeded Market Value, Minimum	50.00%
Greater Than 5% [Member]
Schedule of Investments [Line Items]
Number of Securities	89
Book Value	1,147
Fair Value	$ 890